The transmission of electromagnetic signals from a bore-hole to the earth surface is an effective method of communicating information during various types of drilling operations, such as measuring while drilling (MWD) and/or logging well drilling (LWD). During directional drilling operations, such as boring holes under river beds, subways, unusual earth formations and tapping oil reservoirs, it is particularly important at all times to know precisely the location of the drill bit. A significant effort has been made to develop electrical instruments which are capable of transmitting signals at the drill face or inspection face back to the earth's surface.
A number of systems have been developed which incorporate electromagnetic technology for communicating to the earth surface. For example, in U.S. Pat. No. 5,394,141 to Soulier, described is a system where the lower portion of the drill string is used as an antenna for purposes of transmitting electromagnetic waves carrying information.
In order to enhance communication with the earth's surface, it is preferred to electrically isolate drill string components so that electromagnetic signals can be developed for data telemetry. This is achieved by using a subassembly connector which electrically isolates adjacent drill string components. The isolated components provide the two terminals of an antenna to which an alternating current is applied. This develops the electromagnetic signal for transmission of data to the earth's surface. Examples of such connectors are described in U.S. Pat. No. 6,050,353 to Ryan Energy, U.S. Pat. No. 5,138,313 to Haliburton Company, U.S. Pat. No. 5,163,714 to Geoservice and Canadian patent application 2,151,525 to McAllister Petroleum Services, Ltd.
During directional drilling, the MWD tool measures and transmits basic direction heading data for the purpose of controlling the drilling operation. In addition, the MWD tool collects and stores additional data (i.e. regarding the surrounding geology) that has traditionally been retrieved through a multi-pin connector located at the end of the tool. For this to happen, the tool must be first returned to surface and pulled from the drill collar, after which it is connected to a computer for retrieval of the data. This is a time-consuming and labour intensive process, as it requires disassembly of the drill string. Transmission of this data using traditional data telemetry technology is not practical as the rate of transmission is too slow (i.e. 2 to 3 bits/second). There is clearly a need for an alternate data communication method that allows for higher-speed data retrieval without the labour intensive step of drill string disassembly.
It has also been found that during drilling operations, the drill string is subjected to extreme torsional compression, tension, and bending moments. Such extreme forces can result in connector failure, usually at the weakest point in the subassembly. The connectors of patents and patent application noted above may fail due to overstressing and possibly break up at their weakest point. There is therefore also the need for a connector with enhanced strength characteristics to handle the drilling forces and torques experienced during operation. Consequently, there is a need for a subassembly connector that is capable is withstanding the various forces experienced during drilling, while also electrically isolating adjacent regions to permit the transmission of data via either of the two communication methods discussed above.